Piston bore

ABSTRACT

A piston for a compressor for use in hermetically encapsulated small-type refrigerating machines, with a cylinder-jacket-like sealing and guide surface section ( 1   a   , 2 ) as well as at least two substantially bridge- and cylinder-jacket-sector-like sealing and guide surface sections ( 6 ) of the piston skirt allowing the guidance of the piston in the cylinder as well as sealing the crankcase relative to the sealing or compression chamber and other sections of the piston skirt comprising a recess ( 3, 7 ). In order to provide a piston which despite being partially led out of from the cylinder housing during a working stroke can easily be re-introduced into the same, wherein nevertheless friction- and weight-reducing recesses are provided on the piston skirt and a sufficient guidance of the piston is guaranteed, it is provided that the angle (α) between the connecting lines of the boundary edges ( 9   a   , 9   b ) of each bridge-like and cylinder-jacket-sector-like sealing and guide surface section ( 6 ), which boundary edges extend in the axial direction of the piston, is smaller than 40°, preferably smaller than 30° with the associated rotational axis ( 8 ).

CROSS REFERENCE TO RELATED APPLICATIONS

Applicants claim priority under 35 U.S.C. §119 of AUSTRIAN ApplicationNo. A 1137/2000 filed on 3 Jul. 2000. Applicants also claim priorityunder 35 U.S.C. §365 of PCT/AT01/00217 filed on 3 Jul. 2001. Theinternational application under PCT article 21(2) was not published inEnglish.

The present invention relates to a piston for a compressor for use inhermetically encapsulated small-type refrigerating machines according tothe preamble of claim 1.

The piston of a compressor usually has the task of compressing a workingmedium. The piston performs a reciprocating movement in a cylinder, withthe piston skirt occluding in a substantially sealed manner with thecylinder jacket surface. It thus assumes both guiding as well as sealingtasks.

The friction of the two jacket surfaces on each other leads to anincreased expenditure of energy which the piston needs to exert, thusreducing the overall efficiency of the entire compressor. It istherefore common practice in pistons according to the state of the artto provide recesses on the piston skirt, so that merely a certainsection of the piston skirt will touch the cylinder wall. In thisrespect, the piston skirt comprises in the zone of the recesses asmaller distance from the piston axis than the region in which thepiston skirt occludes in a sealing fashion with the cylinder walls. Therecesses are so deep that supportive oil film can no longer be formed.Friction can therefore be strongly reduced. Moreover, the entire pistonwill become lighter, which also increases the efficiency of thecompressor.

Known pistons for compressors according to the state of the art comprisea sealing and guide surface section in the zone of the piston head whichis arranged in a cylinder-jacket-like fashion and occludes with thecylinder wall in a substantially sealing fashion over the entirecircumference of the piston. The extension of said section in thedirection of movement of the piston is chosen in such a way that asufficient sealing with respect to the working medium is given. Inaddition, such pistons according to the state of the art comprise asecond cylinder-jacket-like sealing and guide surface section which isarranged in the other end region of the piston skirt and again occludeswith the cylinder jacket surface in a substantially sealing fashion overthe entire circumference of the piston. As a result, an alsocylinder-jacket-like recess remains between the two sealing and guidesurface sections.

In the case of a configuration of the piston skirt as in the state ofthe art, there is a problem in the respect that at the lower dead centreof the piston an unthreading may occur of said additional sealing andguide section from the cylinder. In order to reduce the weight of theentire compressor, the cylinder housing is only arranged with such alength as is required by the stroke of the piston. In the process, thepiston moves a bit out of the cylinder at the lower dead centre. Theshorter the cylinder housing, the larger the savings in weight andmaterial and the further the piston will move out of the cylinder at thelower dead centre, however.

In the case of pistons for compressors according to the state of the artthere is a limit which is set by the height of the secondcylinder-jacket-like sealing and guide surface section. The pistoncannot be moved any further out of the cylinder, because in the zone ofthe recess there can no longer be any further guidance of the piston, asa result of which the same would tilt away laterally and the threadingof the lower sealing and guide surface section on the piston into thecylinder would no longer be possible. An extension of the height of thesecond cylinder-jacket-like sealing and guide surface section wouldunnecessarily increase the friction and the weight.

From U.S. Pat. No. 4,350,083 a piston is further known which is providedwith openings on the piston skirt in order to reduce wear and tear. Theyare provided with such a small arrangement with respect to surface areathat a noticeable reduction of friction and thus savings in energy donot occur.

It is therefore the object of the present invention to avoid thisdisadvantage and to provide a piston for compressors which, despitepartially leading the same out of the cylinder housing during a workingstroke, can easily be introduced into the same again. Friction- andweight-reducing recesses are nevertheless provided on the piston skirtand a sufficient guidance of the piston is guaranteed.

This object is achieved in accordance with the invention by thecharacterizing features of claim 1.

As a result of the formation of several narrow, bridge-like sealing andguide surface sections which have the shape of a cylinder jacket surfacesector and extend substantially in the direction of movement of thepiston, the piston can be led to a larger part out of the cylinder inthe zone of the lower dead centre and the projecting part can easily beintroduced back into the cylinder, because sufficient sealing and guidesurface sections are always sufficiently in contact with the cylinderwall during the entire process.

As a result of the merely narrow arrangement of the bridges it ispossible to simultaneously noticeably reduce the friction in the system.Since one piston side is strained more strongly due to the rotationalmovement of the connecting rod, the cylinder-jacket-sector-like sealingand guide surface section can be provided with a stronger arrangementthan the section which is situated in the less strained region of thepiston.

The features of claims 2 and 3 ensure an optimal guidance in thedirection of movement of the piston.

As a result of the characterizing features of claim 4 it is possible toadjust the height of the cylinder-jacket-sector-like sealing and guidesurface sections precisely to the length which the piston projects outof cylinder at the lower dead centre.

As a result of the features of claim 5, said additional sealing andguide surface sections can be optimized further with respect to thefrictional resistance and weight.

The feature of claim 14 describes a preferred embodiment of a piston inaccordance with the invention.

The feature of claim 15 allows additionally reducing the friction andsaving weight at the same time.

A detailed description of the invention and the state of the art isprovided below by reference to the enclosed drawings, wherein:

FIG. 1 shows a piston according to the state of the art;

FIG. 2 shows an axonometric view of a piston in accordance with theinvention;

FIG. 3 shows a further axonometric view of a piston in accordance withthe invention;

FIG. 4 shows an axonometric view of a preferred embodiment of a pistonin accordance with the invention;

FIG. 5 shows an axonometric view of a preferred embodiment of a pistonin accordance with the invention;

FIG. 6 shows a top view of the piston in accordance with the invention;

FIG. 7 and FIG. 8 show an axonometric view of a further preferredembodiment of a piston in accordance with the invention.

FIG. 1 shows a piston for compressors according to the state of the art.Two cylinder-jacket-like sealing and guide surface sections 1 a, 1 benclose an also cylinder-jacket-like recess 3. In the region of therecess 3 one can see the piston pin bore 4 as well as a bore 5 throughwhich a positioning pin (not shown) can be inserted. It is used to fixthe piston pin.

The height of the sealing and guide surface section 1 a is chosen insuch a way that a sufficient sealing is given with respect to theworking medium (not shown). The height of the sealing and guide surfacesection 1 b is merely very low in pistons according to the state of theart, so that when the piston is led out of the cylinder a slight tiltingof the piston would occur at the lower dead centre until the cylinderwall edge touches the recess. The piston would therefore be stuck in aninclined fashion in the cylinder. During the subsequent upwards movementthe sealing and guide surface section 1 b could no longer be introducedinto the cylinder because the edge of the sealing and guide surfacesection 1 b would wedge with the cylinder wall edge.

FIGS. 2 and 3 show axonometric views of a piston in accordance with theinvention which is made of sintered materials or a cast part. Inaddition to the cylinder-jacket-like sealing and guide surface section2, there are two bridge-like sealing and guide surface sections 6adjacent thereto which have the shape of a cylinder jacket surfacesector, extend substantially parallel to the direction of movement ofthe piston and are enclosed by a recess 7. The piston pin bore 4 can beseen in the region of recess 7.

As is shown in FIGS. 4 and 5, a preferred embodiment provides that thebridge-like sealing and guide surface sections 6 having the shape of acylinder jacket surface sector are merely provided with an arrangementof such a height that the piston is not unthreaded from the cylinder atthe lower dead centre.

As a result of the sealing and guide surface sections 6 in accordancewith the invention, a sufficient guidance of the piston in the cylinderis guaranteed, namely not only in cases when the piston does not moveout of the cylinder at all, but even when the piston projects out of thecylinder up to half the entire piston height at the lower dead centre.At the same time, sufficient recesses can still be present in order tominimize the friction and the weight of the piston.

The width of the sealing and guide surface sections 6 in accordance withthe invention are chosen in such a way that an oil film can be formedwhich is adequate for lubrication. According to FIG. 6, the angle αbetween the connecting lines of the boundary edges 9 a, 9 b of eachbridge-like sealing and guide surface sections 6 having the shape of acylinder jacket surface sector with the associated rotational axis 8 issmaller than 40°, preferably smaller than 30°. In this way said sealingand guide surface sections can be kept very small, thus reducing thefriction and the weight of the piston, but still allowing a sufficientoil film to be formed between the sealing and guide surface sections 6and the cylinder wall.

FIGS. 7 and 8 show a further preferred embodiment. The uppercylinder-jacket-like sealing and guide surface section 2 is subdividedby several oil grooves 12. However, in the lower region of thecylinder-jacket-like sealing and guide surface sections 6 in accordancewith the invention a substantially circular-sector-like countersinking13 is provided which further reduces the frictional resistance andadditionally helps reduce weight.

1. A small-type refrigerating machine compressor piston which is freefrom a piston ring groove, with a cylinder-jacket-like sealing and guidesurface section as well as at least two substantially bridge- andcylinder-jacket-sector-like sealing and guide surface sections of thepiston skirt which have continuous surfaces, are flush with an upperface of the piston, and extend substantially parallel to the directionof movement of the piston occluding in a substantially sealing fashionwith the cylinder wall and allowing the guidance of the piston in thecylinder as well as sealing the crankcase relative to the sealing orcompression chamber and other sections of the piston skirt comprising arecess, as a result of which there is a distance between piston skirtand cylinder wall, wherein the angle (a) between the connecting lines ofthe boundary edges of each bridge-like and cylinder-jacket-sector-likesealing and guide surface section, which boundary edges extend in theaxial direction of the piston, is smaller than 40°, with the associatedrotational axis and the bridge- and cylinder-jacket-sector-like sealingand guide surface sections have different have different depths.
 2. Apiston as claimed in claim 1, wherein an end zone of a bridge- andcylinder-jacket-sector-like sealing and guide surface section is flushwith the end of the piston closer to the crankcase.
 3. A piston asclaimed in claim 1, wherein one end zone each of a bridge- andcylinder-jacket-sector-like sealing and guide surface section convergesinto the cylinder-jacket-like sealing and guide surface section of thepiston skirt.
 4. A piston as claimed in claim 1, wherein the bridge- andcylinder-jacket-sector-like sealing and guide surface sections extendfrom the end of the piston closer to the crankcase in the directiontowards the piston head over a length which corresponds at least to thelength which the piston projects out of the cylinder housing at thelower dead center.
 5. A piston as claimed in claim 1, wherein eachbridge- and cylinder-jacket-sector-like sealing and guide surfacesection is interrupted by several point-like recesses.
 6. A piston asclaimed in claim 1, wherein the piston pin bore is arranged in theregion of the recess.
 7. A piston as claimed in claim 1, wherein in theend zone closer to the crankcase a recess is each arranged in the regionof the cylinder-jacket-sector-like sealing and guide surface sections.8. A piston as claimed in claim 1 wherein the angle is smaller than 30°.